In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm.
However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices.
For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the postponement of F2 lithography and the early introduction of ArF immersion lithography were advocated (see Proc. SPIE Vol. 4690 xxix).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens having a numerical aperture (NA) of 1.0 or greater. Theoretically, it is possible to increase the NA to nearly 1.44. It was initially recognized that the resolution could be degraded and the focus be shifted by a variation of water's refractive index with a temperature change. The problem of refractive index variation could be solved by controlling the water temperature within a tolerance of 1/100° C. while it was recognized that the impact of heat from the resist film upon light exposure drew little concern. There was a likelihood that micro-bubbles in water could be transferred to the pattern. The risk of bubble generation could be obviated by thorough deaeration of water and the risk of bubble generation from the resist film upon light exposure is substantially nil. At the initial stage in 1980's of the immersion lithography, a method of immersing an overall stage in water was proposed. Later proposed was a partial-fill method of using a water feed/drain nozzle for introducing water only between the projection lens and the wafer so as to comply with the operation of a high-speed scanner. In principle, the immersion technique using water enabled lens design to a NA of 1 or greater. In optical systems based on traditional refractive index materials, this leads to giant lenses, which would deform by their own weight. For the design of more compact lenses, a catadioptric system was proposed, accelerating the lens design to a NA of 1.0 or greater. A combination of a lens having NA of 1.2 or greater with strong resolution enhancement technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724). Efforts have also been made to develop lenses of NA 1.35.
One candidate for the 32-nm node lithography is lithography using vacuum ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized line edge roughness (LWR) of resist coating, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
The water immersion lithography using a NA 1.35 lens achieves an ultimate resolution of 40 to 38 nm at the maximum NA, but cannot reach 32 nm. Efforts have been made to develop higher refractive index materials in order to further increase NA. It is the minimum refractive index among projection lens, liquid, and resist film that determines the NA limit of lenses. In the case of water immersion, the refractive index of water is the lowest in comparison with the projection lens (refractive index 1.5 for synthetic quartz) and the resist film (refractive index 1.7 for prior art methacrylate-based film). Thus the NA of projection lens is determined by the refractive index of water. Recent efforts succeeded in developing a highly transparent liquid having a refractive index of 1.65. In this situation, the refractive index of projection lens made of synthetic quartz is the lowest, suggesting a need to develop a projection lens material with a higher refractive index. LUAG (Lu3Al5O12) having a refractive index of at least 2 is the most promising material, but has the problems of birefringence and noticeable absorption. Even if a projection lens material with a refractive index of 1.8 or greater is developed, the liquid with a refractive index of 1.65 limits the NA to 1.55 at most, failing in resolution of 32 nm. For resolution of 32 nm, a liquid with a refractive index of 1.8 or greater is necessary. Such a material has not been discovered because a tradeoff between absorption and refractive index is recognized in the art. In the case of alkane compounds, bridged cyclic compounds are preferred to linear ones in order to increase the refractive index, but the cyclic compounds undesirably have too high a viscosity to follow high-speed scanning on the exposure tool stage. If a liquid with a refractive index of 1.8 is developed, then the component having the lowest refractive index is the resist film, suggesting a need to increase the refractive index of a resist film to 1.8 or higher.
The process that now draws attention under the above-discussed circumstances is a double patterning process involving a first set of exposure and development to form a first pattern and a second set of exposure and development to form a pattern between the first pattern portions. See Proc. SPIE, Vol. 5754, p 1508 (2005). A number of double patterning processes are proposed. One exemplary process involves a first set of exposure and development to form a photoresist pattern having lines and spaces at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying another layer of hard mask thereon, a second set of exposure and development of a photoresist film to form a line pattern in the spaces of the first exposure, and processing the hard mask by dry etching, thereby forming a line-and-space pattern at half pitch of the first pattern. An alternative process involves a first set of exposure and development to form a photoresist pattern having spaces and lines at intervals of 1:3, processing the underlying layer of hard mask by dry etching, applying a photoresist layer thereon, a second set of exposure and development to form a second space pattern on the remaining hard mask portion, and processing the hard mask by dry etching. In either process, the hard mask is processed by two dry etchings.
While the former process requires two applications of hard mask, the latter process uses only one layer of hard mask, but requires to form a trench pattern which is difficult to resolve as compared with the line pattern. The latter process includes the use of a negative resist material in forming the trench pattern. This allows for use of high contrast light as in the formation of lines as a positive pattern. Since the negative resist material has a lower dissolution contrast than the positive resist material, a comparison of the formation of lines from the positive resist material with the formation of a trench pattern of the same size from the negative resist material reveals that the resolution achieved with the negative resist material is lower. After a wide trench pattern is formed from the positive resist material by the latter process, there may be applied a thermal flow method of heating the substrate for shrinkage of the trench pattern, or a RELAX method of coating a water-soluble film on the trench pattern as developed and heating to induce crosslinking at the resist film surface for achieving shrinkage of the trench pattern. These have the drawbacks that the proximity bias is degraded and the process is further complicated, leading to a reduced throughput.
Both the former and latter processes require two etchings for substrate processing, leaving the issues of a reduced throughput and deformation and misregistration of the pattern by two etchings. In particular, the double patterning process using a trench pattern requires a more stringent precision of alignment because a misalignment leads directly to a dimensional deviation of line width.
To proceed with a single etching, one method is by using a negative resist material in a first exposure and a positive resist material in a second exposure. Another method is by using a positive resist material in a first exposure and a negative resist material in a higher alcohol of 4 or more carbon atoms, in which the positive resist material is not dissolvable, in a second exposure. In these methods using negative resist materials with low resolution, degradation of resolution occurs.
A method which do not involve post-exposure bake (PEB) and development between first and second exposures is the simplest method. This method involves first exposure, replacement by a mask having a shifted pattern drawn, second exposure, PEB, development and dry etching. Since the throughput is substantially reduced by mask replacement on every exposure, the first exposure is carried out in a somewhat integrated manner before the second exposure is carried out. Then, depending on the holding time between the first exposure and the second exposure, a dimensional variation due to acid diffusion and a profile variation such as T-top profile formation occur. To suppress the T-top formation, application of a resist protective film is effective. Application of a resist protective film for immersion lithography enables a process involving two exposures, one PEB, development and dry etching. First exposure and second exposure may be consecutively carried out by two scanners arranged side by side. This gives rise to such problems as misregistration due to lens aberration between the two scanners and the doubled scanner cost.
If a common resist surface is subjected to exposure under the condition that the first exposure and the second exposure are shifted half-pitch, no pattern is generally formed because the first exposure light offsets the second exposure light. If a contrast enhancement layer (CEL) is applied, the light incident on the resist film becomes nonlinear so that the contrast is increased, and no offset occurs between half-pitch shifted exposures. Then the resist protective film for the double patterning desirably has a function of CEL. Also, if a nonlinear acid generator capable of two-photon absorption is used, it is theoretically possible to carry out double patterning exposures using solely the resist film without CEL. However, an acid generator capable of two-photon absorption has not been reported in conjunction with exposure at wavelength 200 nm or below.
The most critical issue associated with double patterning is an overlay accuracy between first and second patterns. Since the magnitude of misregistration is reflected by a variation of line size, an attempt to form 32-nm lines at an accuracy of 10%, for example, requires an overlay accuracy within 3.2 nm. Since currently available scanners have an overlay accuracy of the order of 8 nm, a significant improvement in accuracy is necessary.
The issue of line edge roughness of resist patterns becomes serious. While a dimensional variation of gate electrodes is a problem that governs the transistor performance, the magnitude of line edge roughness becomes a factor of causing variations to the threshold current of transistors as the feature size becomes increasingly finer. It is intended to reduce the line edge roughness through improvements in the resist material, dry etching technology or photoresist process. For the resist material, a contrast increase by increasing the amounts of both acid generator and quencher added, a reduction of swelling in alkaline developer by incorporating adhesive groups such as fluoroalcohol or lactone, and the like contribute to the roughness reduction. For the photoresist process, flow processes such as thermal flow after development and bromine plasma treatment are effective. However, the flow processes have the problem of size changes due to pattern deformation and pattern shrinkage.